Groundwater contamination related to volatile or semi-volatile compounds is common and can cause serious harm to people, other animals and plants. The United States spends billions of dollars each year treating groundwater contamination. Sources of groundwater contamination include leaking pipelines, chemical spills, landfills and transportation mishaps. Chemicals released at or near the ground surface can migrate through the vadose zone and into the groundwater.
The vadose zone, between the ground surface and the water table, generally consists of subsurface sediments that were deposited in substantially horizontal layers. Porosity levels vary from layer to layer with the conductance of fluids and soil gases usually much greater in the horizontal than in the vertical direction. These conductivity conditions also exist in the saturated materials located below the water table in a zone commonly known as a groundwater aquifer.
Vapors, including gaseous contaminants with moderate to high Henry's Law Constants that are dissolved in the groundwater and volatile liquid contaminants in the groundwater, can escape or evaporate, respectively, back into the vadose zone and become part of the vadose zone soil gas. Conditions that influence the movement and evaporation of contaminants from the groundwater to the vadose zone include a vadose zone with permeable layers, and periodic changes in atmospheric pressure that affect the entire vadose zone.
Prior known remediation systems and methods to treat contaminated groundwater include two main types: in-situ treatments that primarily treat contamination below ground surface, and treatment processes that occur mostly above ground surface. All of these prior known groundwater remediation systems and methods require expensive groundwater treatment wells and above ground infrastructure to support or control the groundwater remediation activities. These prior known remediation systems all include cased wells that are drilled through the vadose zone and into the contaminated aquifer. The cased wells have casings that have smaller diameters than the boreholes and often extend several feet above ground.
The cased well casings in the prior known groundwater remediation systems and methods have a screen section that extends upwardly a selected distance from the bottom end of the borehole. These cased wells generally have engineered porous fill material located between the screen and the borehole wall from the bottom end up to slightly above the top of the screen section. Above the fill material substantially impermeable materials seal the space between the casing and the borehole wall. Fluids can only travel up or down the inside of the casing from the screened section to treatment systems located above the ground surface and usually at some distance from the treatment wells.
The prior known remediation systems include various blowers, pumps and other components that are often expensive. Power to run the mechanical and electrical components, as well as the ongoing equipment maintenance activities adds to the cost. In addition, regulatory requirements in many states require that treatment wells and other components be removed after the systems are no longer in use, adding to the costs associated with operating these prior known remediation systems.
The time needed to complete the groundwater remediation often takes years to decades of time with prior known remediation methods and systems. The wells in prior known remediation systems are also limited in location as the above ground components are restricted to areas where near ground obstructions are not a hazard to transportation systems or pedestrians. During the remediation period impacted properties rarely can be used for any other purpose but the operation of the treatment systems, and many of these properties must have security fencing to protect and prevent unwanted access to the remedial equipment or the treatment wells.
Contaminants from the groundwater will migrate through the vadose zone if a pathway conducive to this movement exists. Such a pathway must include a porous transport route from the groundwater, through the horizontal layers in the vadose zone and into the atmosphere. In addition, differences must exist between the pressures of the contaminants and that of the atmosphere in order to support the transport of contaminants upwards and through the conductive pathway. If a porous pathway does not exist, the contaminants will remain confined below ground in semi-equilibrium with the contaminants present in the groundwater. Movement of these materials is primarily by slow diffusion processes toward the ground surface. The cased wells of prior known groundwater remediation systems are constructed with only the screen section at the bottom of the casing, substantially limiting migration of vapors through the vadose zone to the surface.